This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying regions of rock formations having significant water saturations from which hydrocarbons may be produced without significant attendant water production.
Subsurface reservoirs of natural gas and petroleum, hereinafter referred to generically as "hydrocarbons" are typically found trapped in permeable geological strata beneath a layer of impermeable strata material. A hydrocarbon will "float" upon any ground water present, although typically a transition zone will exist between the two fluids due to the water being raised by capillary action of the permeable strata material. In some regions, impermeable layers may be relatively closely stacked atop one another trapping thin zones of what may be essentially hydrocarbons, essentially water or mixed hydrocarbons and water. A wellbore dropped through the formation and various layers may produce water if tapped in a transition region or mixed hydrocarbon and water zone. The cost of transporting, separating and disposing of the attendant water adds sufficiently to production costs that hydrocarbon reservoirs have often been left untapped where it is expected or believed they would produce an excessive amount of attendant water.
Water saturation present at various levels of a formation is typically determined from interpretation of conventional electrical (i.e. resistivity) logs taken through a borehole drilled through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation set forth in "The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics", Trans. AIME, Vol. 46, pp. 54-62, 1942, by G. E. Archie. This equation is expressed as follows: EQU S.sub.w.sup.n =R.sub.w /.phi..sup.m R.sub.t ( 1)
Where "S.sub.w " is the fractional water saturation (i.e. free and bound water of the formation expressed as a percent of the available pore space of the formation), "R.sub.w " is the formation water resistivity, ".phi." is the formation porosity, "R.sub.t " is the formation resistivity indicated by the resistivity log, "n" is the saturation exponent and "m" is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict fractional water saturation S.sub.w from the log-indicated resistivity R.sub.t, using the equation in any of its forms.
Archie defined two quantities that provided the basis for his water saturation equation (1). The first quantity is the formation factor F which defines the effect of the rock matrix on the resistivity of water as follows: EQU F=R.sub.o /R.sub.w ( 2)
where
R.sub.o =resistivity of water-saturated rock and PA1 R.sub.w =water resistivity.
Archie reasoned that for a given value of R.sub.w, the formation factor F would decrease with increasing porosity, .phi., to some exponent m: EQU F=1/.phi..sup.m ( 3)
This porosity exponent m has also become known as the cementation exponent. Thus Archie provided a useful characterization of a rock fully saturated with a conducting brine in terms of the water resistivity Rw, porosity .phi. and a rock parameter m. It is important to note that Archie assumed all conductance to be in the brine.
The second quantity is the resistivity index I defined as the ratio of the resistivity of a rock partially saturated with water and hydrocarbon R.sub.t, to the same rock saturated fully with water, R.sub.o, as follows: EQU I=R.sub.t R.sub.o ( 4)
Archie reasoned that as the water saturation decreased (i.e. hydrocarbon saturation increased) the resistivity R.sub.t and hence I would increase to some exponent n: EQU I=1/S.sub.w.sup.n ( 5)
where S.sub.w =volume of water in pores/total volume. This exponent n has become known as the saturation exponent. It is again important to note that Archie assumed all conductance to be in the brine and further that all pores within the rock have the same water saturation S.sub.w.
It is these two equations (3) and (5) for the formation factor F and resistivity index I respectively that Archie combined to provide the water saturation expression S.sub.w of equation (1). Certain logs provide porosity .phi., water samples provide the best values for R.sub.w, and the cementation exponent m and saturation exponent n are obtained by electrical measurements on core samples.
Standard practice is to measure rock sample resistivities R.sub.o and R.sub.t for a number of water saturations and to plot the logarithm of I versus the logarithm of S.sub.w. Such a logarithmic plot is a straight line with slope of -n. This plot, however, assumes that all rock pores are desaturated equally, all resistivities for partial water saturation are measured under uniform fluid distribution conditions throughout the rock sample, and all conductance is in the brine. If uniform fluid distribution has not been reached throughout the rock sample, then the value of the measured resistivity index I will not be correct and therefore the value of the saturation exponent n will not be the value of n that is characteristic of the rock. It is therefore the specific objective of the present invention to provide a method for measuring electrical resistivity of a rock sample that will yield a correct value for the saturation exponent n even under conditions in which water saturation has not reached uniform fluid distribution throughout the rock sample. Present methods cannot make such an identification, but merely rely on waiting periods after each new partial water saturation is effected in the rock sample over which it is assumed that uniform fluid distribution has been reached so that a resistivity measurement can be made.